1. Field of the Invention
The present invention is a continuation-in-part application of U.S. patent application Ser. No. 644,446, now abandoned, and relates to a cermet cutting tool, and process for producing the same, in which a hard coating layer is formed onto the surface of a titanium carbo-nitride based cermet substrate. In particular, the present invention pertains to improvements for enhancing the chipping resistance and wear resistance of the blade member.
2. Background Art
Conventionally, cermets, containing at least one metal of carbide, nitride and carbo-nitride of titanium (Ti), zirconium (Zr), hafnium (Hf), tantalum (Ta), niobium (Nb), tungsten (W), molybdenum (Mo) and chromium (Cr) as a hard phase-constituting component and iron family metals such as cobalt (Co), nickel (Ni) and the like as binder metals, have been widely used to form blade members or inserts for use in finish cutting of steel.
In recent years, the aforementioned cermets possessing surface coatings composed of TiC, TiCN and the like, which further improve wear resistance, are being produced.
For example, Japanese Patent Application First Publication No. Sho 54-117510 discloses a cermet cutting tool which is formed by applying to a titanium carbo-nitride based cermet substrate using a chemical vapor deposition process, a hard coating layer of thickness 0.5.about.20 .mu.m which is formed from at least one layer of a compound selected from the group consisting of titanium carbide (TiC), titanium nitride (TiN), titanium carbo-nitride (TiCN), titanium carbo-oxide (TiCO), titanium carbo-oxi-nitride (TiCNO) and aluminum oxide (Al.sub.2 O.sub.3). In addition, as the substrate, use of a TiCN based cermet in which the incorporated amount of binding phase-constituting component, Co or Ni, is comparatively higher in the surface layer portion when compared to the substrate interior, is known.
In this case, as the chemical vapor deposition process, a conventional process is employed in which a TiCN coating layer is formed onto the cermet surface using a mixed gas composed of 4% of TiCl.sub.4, 3% of CH.sub.4, 4% of N.sub.2, and 89% of H.sub.2 (% by volume) at a temperature of 1000.degree. C. and a pressure of 100 Torr inside the chemical vapor deposition apparatus. The reaction in this case is represented by the following formula: EQU TiCl.sub.4 +CH.sub.4 + 1/2N.sub.2 +H.sub.2 .fwdarw.TiCN+4HCl+H.sub.2
However, when the aforementioned prior art cermet cutting tools are used in steel and cast-iron high-speed cutting inserts, chipping of the cutting insert occurs easily due to the lack of toughness of the hard coating layer, and considerable wear occurs due to this chipping, thereby exhausting the tool life in a comparatively short time.
One source generating the aforementioned problems lies in the fact that the binding phase-constituting component (Ni or Co) diffuses into the TiCN layer during chemical vapor deposition, creating a materially inferior TiCN layer. In other words, the iron family metals such as Co and Ni which comprise the binding phase-constituting components, due to the high coating temperature (1000.degree. C.) of prior art chemical vapor deposition processes, diffuse into the TiCN layer and cause loss of the original wear resistance, resulting in formation of a coating layer possessing a low wear resistance. However, if chemical vapor deposition is attempted at a temperature of 900.degree. C. or less where it is difficult for the binding phase-constituting component to diffuse into the TiCN layer, the activity of reaction gases such as methane, nitrogen and the like is reduced, and the reaction does not occur, which results in the TiCN coating layer being unobtainable. Consequently, under the aforementioned temperature conditions of the prior art, a coating layer cannot be formed.
The inventors of the present invention have conducted various research in order to obtain a TiCN coating layer with superior wear resistance by efficient chemical vapor deposition of a TiCN layer at a temperature of 900.degree. C. or less, where it is difficult for the aforementioned binding phase-constituting component to diffuse into the TiCN layer. As a result, when adding acetonitrile (CH.sub.3 CN) to the mixed gas used in the chemical vapor deposition, highly efficient formation of a TiCN coating layer is possible even when chemical vapor deposition is performed at a temperature of 900.degree. C. or less: the TiCN coating layer formed in this manner, due to non-diffusion of the aforementioned binding phase-constituting component, was confirmed by observation to possess a superior wear resistance.
Furthermore, the inventors of the present invention, in order to further improve the wear resistance of the TiCN layer, have continued their research. As a result, they have discovered a novel development in that when chemical vapor deposition is performed using a reaction gas composed of 1.about.5% of TiCl.sub.4, 0.1.about.1% of CH.sub.3 CN, 0.about.25% of N.sub.2 with the remaining portion being composed of H.sub.2, under a reaction temperature of 800.degree..about.900.degree. C. and a reaction pressure of 30.about.200 Torr, at least one portion of the TiCN layer comprises a longitudinal growth crystal structure, which is a change from the normal granular crystal structure, resulting in further improvement of both the chipping resistance and the wear resistance.